Generally, a magnetron used in an electronic appliance such as microwave oven, as shown in FIG. 1, comprises an anode 10 for generating microwaves as well as heat of high temperature, a plurality of heat radiation plates 11 mounted around the anode for dissipating the heat generated by the anode, a yoke 16 for enclosing the heat radiation plates, a seal 17 connected to the upper and lower ends of the anode, and a ring-shaped permanent magnet 18.
In such a conventional magnetron, when the anode is applied with a voltage, the motion of thermally generated electrons generates microwaves outputted through the output means 19, in which case a portion of the microwaves is dissipated in the form of heat. Namely, the heat of high temperature generated by the anode 10 is in part dissipated through the heat radiation plates 11 and in part through the seal 17 and permanent magnet 18, when a blowing fan (not shown) drives cooling-air into the gaps between the yoke and heat dissipation plates so as to prevent the temperature increase of the anode and the capability degradation of the permanent magnet. In this case, when the cooling-air flows through the inside of the heat radiation plates 11, the rear side of the anode 10 is not effectively cooled by the cooling-air compared to the front side because, as shown in FIG. 2, it is hardly possible to obtain uniform cooling of the anode.
In order to obtain uniform cooling of the anode there has been proposed a guide wall 15 provided on the heat radiation plates near the flange 12 for supporting the anode 10, as shown in FIG. 3, which is disclosed in U.S. Pat. No. 4,794,304. In this case, when the cooling-air is driven through the gaps between the heat radiation plates by a blowing fan, the pressure difference between the rear and front sides of the cylindrical anode causes the air-separation in the rear region of the anode to push the main stream of the cooling-air away from the rear side of the anode, Hence the cooling effect of the rear side of the anode is considerably worse than that of the front side, so that there occurs a temperature difference of a few .degree. C. to several tens of .degree. C. between the rear and front sides of the anode. This temperature difference degrades the output efficiency of the magnetron and adds thermal distortion of the anode, thus considerably reducing the effective life of the magnetron.